The invention pertains to a method for operating an internal combustion engine of a motor vehicle with a butterfly valve, where the butterfly valve is opened and the fuel supply is cut off when the vehicle is in overrun mode. The invention also pertains to a system for actuating a butterfly valve of an internal combustion engine of a motor vehicle so that the butterfly valve can be adjusted to a value based on the position of the gas pedal.
In a motor vehicle with, for example, an NOx storage catalyst, the butterfly valve is opened completely and the fuel supply cut off during the times that the vehicle is in overrun mode. The purpose of this is to increase the rate of flow through the catalyst system and thus to support the cooling of the catalyst. The NOx storage catalyst is thus cooled more quickly to its working temperature. With this increased flow technique, three-way catalysts can also be cooled more effectively with large, nearly pollutant-free streams of exhaust gas. The heat exothermically released by the catalyst as a result of peak pollutant levels at the beginning of a fuel supply cutoff phase can be dissipated quickly by the following stream of cooling gas.
During the operation of the internal combustion engine of a motor vehicle with an opened butterfly valve in overrun mode, a small drag torque of the internal combustion engine can lead to a noticeable change in the driving behavior. In addition, when fired operation begins again, the driving behavior of the vehicle becomes uncomfortable and unpleasant for the driver and the passengers. When, for example, there is low load demand at the end of the overrun phase and the butterfly valve is actuated too late and/or too slowly, the decrease in the filling process will be delayed to a corresponding extent, and the motor vehicle will present the driver with unexpected responses, such as an increase in the power output of the internal combustion engine, even though the driver has not depressed the gas pedal with the goal of obtaining a significant increase in power but merely of ending the overrun phase and arriving in an operating state with a certain small amount of drive power.
The present invention is therefore based on the task of providing a system of the type indicated above which avoids the disadvantages indicated above and which makes available an overrun mode with an open butterfly valve which does not cause any changes in the driving behavior of the motor vehicle or any loss of driving comfort.
It is thus provided according to the invention in a method of the type indicated above that a first value for a position of the butterfly valve during the overrun phase is determined as a function of a motor vehicle speed and an engine rpm value at the beginning of the overrun phase, and that the butterfly valve is adjusted to this value during the overrun phase.
This offers the advantage that, no matter what the driving situation, a transition from overrun mode to fired mode with an open butterfly valve or from fired mode to overrun mode with an open butterfly valve will always occur without any noticeable change in the driving properties of the motor vehicle and with an appropriate drag moment during the overrun phase.
For an especially comfortable transition from overrun mode to fired mode, a second value for the position of the butterfly valve is determined during the overrun phase on the basis of the speed of the vehicle and the engine rpm""s in such a way that this second value corresponds to the position of butterfly valve for uniform travel at the currently determined vehicle speed. The butterfly valve is adjusted to this second value at the end of the overrun phase.
An especially effective way of setting the butterfly valve to the most probable anticipated value, that is, an especially effective way of calculating a value for the position of the butterfly valve after the end of the overrun phase, is to determine the speed at which the driver""s foot is repositioned from the brake pedal to the gas pedal, the second value of the position of the butterfly valve being corrected in accordance with this foot-repositioning speed. The second value for the position of the butterfly valve is preferably corrected on the basis of the foot-repositioning speed in such a way that, when the foot is repositioned quickly, the second value of the position for the butterfly valve is changed in the direction of higher power output from the internal combustion engine, and when the foot is repositioned slowly, the second value of the position of the butterfly valve is changed in the direction of lower power output from the internal combustion engine.
An even more accurate estimate of the probable load which will be demanded by the driver after the end of overrun mode can be obtained by determining the speed at which the gas pedal is actuated at the end of an overrun phase and by correcting the second value for the position of the butterfly valve in correspondence with the gas pedal actuation speed. The second value for the position of the butterfly valve is advisably corrected on the basis of the gas pedal actuation rate in such a way that, when the gas pedal is actuated quickly, the second value for the position of the butterfly valve is changed in the direction of higher power output from the internal combustion engine, and when the gas pedal is actuated slowly, the second value for the position of the butterfly valve is changed in the direction of lower power output from the internal combustion engine.
The two last-mentioned elaborations of the invention include the driver""s behavior in the calculation of the probable future value for the position of the butterfly valve, which leads to an especially good estimate or calculation of this value.
To prevent the action of a brake booster from decreasing too quickly in overrun mode as a result of the open butterfly valve, a third, priority value for the position of the butterfly valve is adjusted when the brake booster signals that it requires assistance. This third value is selected so that sufficient negative pressure is available to assist the brake booster.
So that, within in a finite period of time after the end of overrun mode, the butterfly valve returns to the actual position based on the position of the gas pedal, the system switches the position of the butterfly valve from the calculated and possibly corrected second value back to the nominal value of the engine control system after a predetermined period of time. It is advisable for this predetermined time to be taken from, for example, a map of characteristic values on the basis of the second value for the position of the butterfly valve and the rpm value of the engine at the moment in question. Any difference which may occur during the predetermined time between the value for the position of the butterfly valve based on the gas pedal position and the calculated and possibly corrected second value for the position of the butterfly valve is compensated preferably by a change in the ignition angle.
In a system of the type described above, furthermore, a calculator for generating a calculated a value for the position of the butterfly valve is also provided, where the system adjusts the butterfly valve to the calculated value instead of to the value based on the position of the gas pedal for a predetermined time after the end of the overrun phase.
This offers the advantage that, no matter what the driving situation, a transition from overrun mode to fired mode with an open butterfly valve or from fired mode to overrun mode with an open butterfly valve will always occur without any noticeable change in the driving properties of the motor vehicle and with an appropriate drag moment during the overrun phase.
For an especially comfortable transition from fired mode to overrun mode, the system has a first device for determining a first calculated value for the position of the butterfly valve during overrun mode. This value can be taken, for example, from a map of characteristic values on the basis of the speed of the vehicle and the rpm""s of the engine at the beginning of the overrun mode.
An especially effective way of adjusting the butterfly valve to the most probable anticipated value, that is, of calculating a value for the position of the butterfly valve after the end of overrun mode, is to provide the system with a second device for determining a second calculated value for the position of the butterfly valve after the end of overrun mode, which value can be taken, for example, from a map of characteristic values on the basis of the speed of the vehicle and the rpm""s of the engine at the end of overrun mode.
A good-quality estimate of a most probable position of the butterfly valve to be expected after the end of overrun mode can be obtained by taking into account the behavior of the driver. The second device preferably comprises a sensor for determining the speed at which the driver repositions his foot from the brake pedal to the gas pedal. This second device then corrects the second calculated value of the position for the butterfly valve in correspondence with the foot-repositioning speed. For example, the second device corrects the second calculated value of the position of the butterfly valve on the basis of the foot-repositioning speed in such a way that, when the foot is repositioned quickly, the corrected second calculated value for the position of the butterfly valve corresponds to a value of higher power output from the internal combustion engine, whereas, when the foot is repositioned slowly, the second corrected value for the position of the butterfly valve corresponds to a value of lower power output from the internal combustion engine.
Alternatively or in addition to a sensor for detecting the foot-repositioning speed, the second device comprises a sensor for determining the speed at which the gas pedal is actuated and/or for determining an instantaneous speed of the vehicle. The second device then corrects the second calculated value for the position of the butterfly valve in correspondence with the gas pedal actuation speed or the vehicle speed by the use of a map of characteristic values. The second device corrects the second calculated value for the position of the butterfly valve on the basis of the gas pedal actuation speed, for example, in such a way that, when the gas pedal is actuated quickly, the corrected, second calculated value of the position of the butterfly valve corresponds to a value of higher power output from the internal combustion engine, and when the gas pedal is actuated slowly, the corrected second value for the position of the butterfly valve corresponds to a value of lower power output from the internal combustion engine.
To prevent the action of a brake booster from decreasing too quickly as a result of the open butterfly valve in overrun mode, the system has an input connected to a control unit for a brake booster, where the system sets a calculated priority value for the position of the butterfly valve when requested by the control unit of the brake booster in such a way that sufficient negative pressure is available for assisting the brake booster.
So that the position of the butterfly valve can return to the actual value based on the position of the gas pedal within a finite period of time after the end of overrun mode, the system has a time control unit, which switches the position of the butterfly valve from the calculated value to the value based on the gas pedal position after a predetermined amount of time has elapsed from the end of overrun mode. It is advisable for the time control unit to take the predetermined time from a map of characteristic values on the basis of the calculated value for the position of the butterfly valve and an instantaneous engine rpm value.